1. Field of the Invention
This invention relates to a gradient index type single lens suitable as the collimator lens of a semiconductor laser or the pickup objective lens or the like of an optical disc.
2. Description of the Prior Art
SELFOC lens (trade name) is well known as a lens having an index gradient in direction perpendicular to the optic axis thereof, i.e., so-called radial gradient index lens, and has been used as an erect one-to-one magnification imaging element in a copying apparatus or the like.
In recent years, attempts have been made to use this gradient index type lens as the pickup objective lens of a digital audio disc or the like. The use of a plano-convex gradient index type lens is shown in the 4th topical meeting on gradient-index optical imaging systems. However, in the single lens shown therein, only the correction of spherical aberration which is an on-axis aberration is considered. In contrast, where such lens is actually used as a pickup objective lens or a collimator lens, not only the on-axis aberration but also off-axis aberration must be well corrected.